Playback from bag files results in different number of frames inspite of set_real_time=False

[vidullan]

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Required Info
Camera Model	{ D435 }
Operating System & Version	Ubuntu 18
Kernel Version (Linux Only)	5.3
Platform	PC
SDK Version	2.34.0.1454
Language	python3

Issue Description

I've recorded bag files using the real sense viewer in Windows 10 and am trying to extract frames from it in python3 in ubuntu.

A) The issue is even though I'm in playback mode with real time set to false I get inconsistent number of frames read from the bag file. For example, on the exact same file on different runs I get anywhere between 728 frames and 300 frames.

Is there no way to read all the available frames consistently since this is a pre-recorded file? The information is obviously there as rerunning the same snippet seems to extract all the frames sometimes, other times some frames are dropped, and sometimes I get many repeated frames.

(I'm using the timestamp to determine end of file since using repeat_playback=false means wait_for_frames will hang)

Here is the relevant code snippet:

Create pipeline

pipeline = rs.pipeline()

Create a config object

config = rs.config()
config.enable_device_from_file(bag_path)
profile = pipeline.start(config)
playback = profile.get_device().as_playback()
playback.set_real_time(False)

frame_num = 0
ts_prev = 0
while True:
frames = pipeline.wait_for_frames()
ts = frames.get_timestamp()
if ts < ts_prev:
break
else:
ts_prev = ts
frame_num +=1

B) If I run that code in a loop to read multiple bag files I get a segmentation fault arising from the realsense library. Sometimes it happens after 1 bag file, sometimes after 10-15 bag files. Is there a known bug in the library? I've also tried closing the pipeline after each file to no avail.

[MartyG-RealSense]

Hi @vidullan You are not the first person to receive inconsistent frame counts from a bag file when reading it with Python with set_real_time = false. During my research into your problem, the most common advice for this problem from RealSense support team members was to check the frame count with a program called rosbag-inspector to find out what the correct number of frames in the bag file should be when reading the count with your own Python code.

https://github.com/IntelRealSense/librealsense/tree/master/tools/rosbag-inspector

One of the support team members suggested the following method for checking frame count with rosbag-inspector:

"Open the target bag file, and look at the topics /device_0/sensor_0/depth_0/image/data or /device_0/sensor_1/color_0/image/data and look at the count".

The link below directs to a RealSense user's Python script that makes use of set_real_time when extracting frames from a bag. Also check out the script in the comment below it. Hopefully these will provide some useful guidance for your own Python frame extraction script.

#3682 (comment)

[vidullan]

Thanks for the reply @MartyG-RealSense and the link.

Upon further investigation, the problem seems to be the API repeating frames an arbitrary number of times. Using the timestamp associated with each frame to reject duplicate frames seems to generate the correct number of frames consistently.

[MartyG-RealSense]

I'm pleased that you found a solution. Do you require further assistance, please?

[vidullan]

@MartyG-RealSense If possible could you shed insight into why I might be having segmentation faults when trying to read multiple bag files in a loop? Is it also a known issue?

All it is is the same code that extracts images from the bag file modified to loop over a list of bag files. Sometimes it runs on just 1 file before throwing a segmentation fault other times I get through 5-10 files. Seemingly random occurrence.

[MartyG-RealSense]

There was a past case where a C++ user was experiencing segmentation errors when playing multiple bag files, so it may not be a problem that only affects Python.

#2693

A RealSense support team member provided feedback in that discussion about a method of looping that worked for them whilst running a loop test.

#2693 (comment)

The mechanics of such loops and why segmentation faults may occur during them are outside of my programming knowledge, so I recommend posting a new issue on the GitHub asking about that particular subject if you would like to explore it further.

[vidullan]

@MartyG-RealSense thanks for the link and all your help. I'll close this issue for now.

[MartyG-RealSense]

Thanks very much for the update - good luck!